1. Field of the Invention
The present invention relates to a projection type exposure apparatus for use in manufacture of semiconductor devices.
2. Related Background Art
The manufacturing method of semiconductor devices generally includes a lithography process in which a wafer is subject to a repeating cycle of resist coating-alignment-exposure-chemical treatment. With the tremendous increase of the degree of integration of semiconductor devices in recent years, a reducing projection type exposure apparatus has been introduced for carrying out the steps of alignment and exposure in the lithography process. The apparatus has now been widely used for this purpose.
The reducing projection type exposure apparatus is required to have a projection lens of high resolving power. Through the high resolution projection lens, an image of a pattern on a reticle is projected onto a wafer. The reticle is a photo mask composed of a light-transmissive part and a light-blocking part. The wafer has a size of about 75 mm to 150 mm in diameter and is placed on a moving stage. The pattern image is projected in a square exposure field of 10 mm.times.10 mm to 20 mm.times.20 mm on the wafer. Since the stage-shift and the exposure are repeated a number of times for one and same wafer, the alignment, exposure and stage-shift must be carried out rapidly. At the same time, it is essential to attain the alignment at a high degree of accuracy corresponding to the high resolving power of the projection lens.
For the alignment step using the reducing projection type exposure apparatus, the alignment is carried out according to the so-called die-by-die alignment method. According to the alignment method, at every exposure of a pattern image through the projection lens, an image of an alignment mark is also projected on the wafer so that by the chemical treatment after exposure an alignment mark may be formed.
For the next exposure, this alignment mark on the wafer is aligned with an alignment on the next reticle. This alignment is carried out every time after exposure.
Since the alignment mark on the reticle is aligned with the alignment mark on the wafer through an aberration-compensated projection optical system, it is desirable that the operator can check the state of the alignment also during the printing exposure. However, in case of the known TTL alignment optical system for alignment through a projection lens, the check of the alignment is impossible during the exposure. This is because a part of the alignment optical system is disposed in the light path for exposure and at the time of exposure the part is retracted to a position out of the exposure light path. In addition, the known alignment optical system has a very complicated construction.
A further problem is caused by the illumination light used to illuminate the alignment marks in the prior art apparatus. In many of the above-mentioned prior art apparatus, to illuminate both the alignment marks on the reticle and on the wafer there has been used illumination light of a wavelength which is near the wavelength region of the light for exposure. This fact has produced an unfavorable effect on the relation between the layer of resist (photosensitizing agent) coated on the wafer and the light for alignment.
The ordinary resist is photo-sensitive to a wide range of wavelengths including the wavelength near the exposure wavelength. Therefore, when the wavelength used for alignment is near the wavelength for exposure, the resist on the alignment mark is unfavourably subjected to photochemical change by the exposure light at the time of exposure. Because of this, reprinting of the alignment mark is needed at every time after the exposure step. This is not only troublesome but also a cause for the decreasing of the alignment accuracy. Furthermore, the alignment detection signal is rendered unstable. Since the alignment mark on the wafer is observed through the resist layer, the qualitative change of the resist by the exposure light produces a change in state of observation (mainly contrast of alignment mark). The state of observation after the exposure is no longer the same as that before the exposure, which renders the alignment detection signal unstable.
To solve the problem, a multi-layer resist and a dye-containing resist CED (Contrast Enhanced Litography) have already been proposed. The multi-layer resist is a resist so formed that under the condition of interference to the exposure wavelength it is non-reflective. The dye-containing resist CEL uses a sensitizer having a light-absorbing layer. These particular resists also have a disadvantage that the alignment mark on the wafer is hardly observed with the alignment wavelength near that of exposure.